The present invention relates generally to a welding apparatus, and more particularly to an apparatus for welding rolled seams by means of a rotating electrode.
It is known to weld rolled seams by means of an electrode roller which rotates, either as the workpiece passes beneath the electrode or as the electrode passes over the workpiece, or both. The welding current that is supplied to the roller electrode is stationarily produced by means of a transformer and subsequently arranged rectifier unit that are connected to a three-phase net supply, and is then transmitted to the rotating electrode roller. The transmission is accomplished by means of slip rings, or else by means of slide or mercury bearings. These prior-art proposals are not entirely satisfactory, and the manner of transmitting current to the roller electrode becomes the more difficult and results in the higher electrical losses, the higher the amperage of the direct welding current is required to be, in order to obtain optimum welding efficiency. In the case of extremely high amperage to be transmitted, the use of slip rings has the disadvantage that a large number of such slip rings is required. The use of slip rings as well as the use of slide bearings both have the disadvantage which is also common to all rotating types of contacts in an electrical system, namely the current transmission is undefined and takes place from point to point, so that a substantial wear develops which drastically reduces the lifetime of the equipment.
In the case of mercury transmission, high losses of current are experienced, and in addition a sealing problem is experienced, and substantial maintenance difficulties exist, which are not least crossed by the potential health hazards resulting from mercury vapors.
Because of the aforementioned drawbacks, the amperage that could be transmitted in this type of equipment was strictly limited, not only for practical but also for economic reasons.
In many instances, however, it is desired to be able to use substantially higher amperage currents for welding operations, than could be transmitted with the prior-art equipment. For example, high-amperage currents in connection with the use of direct current welding current are desired to be used in order to obtain high welding speeds on the order of 100-120 meters per minute and more, while nevertheless obtaining uniform and high-quality welds. This is particularly true in the case of the continuous welding of longitudinal seams in the case of tubes or pipes having a small wall thickness. In order to obtain maximum welding effectiveness, while at the same time also producing high-quality uniform welds, and to be able to do this economically and technically with a reasonable expenditure, direct current is required having extremely high amperage, low voltage and low harmonics. However, this has always encountered great difficulties, since the transmission by means of rotating contacts of welding currents in excess of 100,000 amperes and a few volts may result in transmission losses of up to 50 percent of the total current. Adding to this the additional losses in the rectifier which are approximately of the same order of magnitude, it is clear that economic considerations have very often necessitated the use of lower amperages and consequently to accept the decrease in welding efficiency which results from such lower amperages.